A modest but extraordinarily graceful film about what people need from each other, and the limits of what they can give of themselves.

Debra Granik is drawn to stories about survivors — stories about people who don’t fit into the one that America likes to tell itself, but are no less valuable for that. They live in the margins, far removed from the capitalistic power of what Ken Kesey once called the Combine. Some of them, like the destitute 17-year-old Jennifer Lawrence played in “Winter’s Bone,” were simply born there. Others, like the tender but troubled Vietnam vet at the heart of Granik’s 2014 documentary “Stray Dog,” have been too close to the big machine, and can’t stomach the idea of going anywhere near it again.

The terse and wary father in Granik’s latest film most definitely falls into the latter category. In fact, that’s all we really know about him. A man as humble and inscrutably compassionate as the movie around him, Will (Ben Foster) doesn’t like to say very much or make himself vulnerable, not even to the tween daughter he’s raising by himself in the verdant sogginess of Portland’s Forest Park. Tom (New Zealand actress Thomasin Harcourt McKenzie) is probably aware that her dad served overseas, but she may not know where, or why his experience inspired him to raise her off the grid.

They forage their own food, sleep in the same tent, and huddle together when the wolves sniff around at night; Will would never dream of hurting or abusing Tom, he’s just trying to keep her safe in the only way he knows how. It’s worked for this long, but their world is forcibly relocated behind walls after a jogger spots Tom hiding in the woods and decides to alert the authorities. Just like that, these two outsiders are swept into the system, seen after more than a decade of living invisibly. “We can still have our own thoughts,” Will tells his kid, but she’s the only one who seems to believe that.

Adapted from the 2009 Peter Rock novel “My Abandonment” and refitted with a less violent title that suits this version’s gentle soul, “Leave No Trace” sprouts into a modest but extraordinarily graceful film about what people need from each other, and the limits of what they can give of themselves. Like a riff on “Captain Fantastic” that actually takes place on planet Earth (Tom reads the encyclopedia in lieu of using the internet, and is naturally more advanced than the average kid her age) Granik’s unhurried drama recognizes the appeal of living outside without romanticizing that life; it views self-sufficiency as a survival technique, not an aspiration.

Without fail, every single person who Will and Tom encounter on their vagabond journey to nowhere tries to help them out (how refreshing to see that happen in a film that feels so realistic). From the Oregon state agent who finds them a new place to live, to the neighborhood boy (“Winter’s Bone” alum Isaiah Stone) who introduces Tom to an adorable rabbit named “Chainsaw,” to the lonely truck driver who makes sure the girl isn’t in any danger before he agrees to let she and her father hitchhike away from their latest attempt at a home, Granik shows us an unrecognizably kind and merciful country. The big machine may be broken, but its individual parts still work just fine on their own.

“Leave No Trace” isn’t always a a gripping adventure, and its vagaries occasionally become so broad that Tom and Will lose the specificity that binds them to each other, but the film is consistently at its best when it’s attuned to a quiet sense of harmony between all things. In a largely apolitical story that’s only discernible issue is our collective apathy towards vets, Granik’s unobtrusive direction helps avoid an “us vs. them” mentality (and overpowers Will’s paranoia) through more organic expressions of unity — the healing power of animals; Dickon Hinchliffe’s minimalistic score; how an empty house can keep two strangers warm.

Alas, Will is a hard man to reach. The more compassionate the world is to he and his daughter, the less he seems to trust it. Foster’s natural volatility has seldom been put to better use, as you keep waiting in vain for Will to explode. Tom does as well, the stoic young girl having spent her whole life walking barefoot on eggshells. She’s not afraid of her dad so much as she’s afraid of not understanding him, or what he wants for them both, and that fear grows more real by the day. McKenzie is a brilliant find, her mousy voice hiding an enormous strength, like a thin tarp stretched across a bottomless pit.

It’s Will who becomes his daughter’s greatest weakness, and “Leave No Trace” is most affecting as a portrait of a father reckoning with that sad fact. Is their relationship healthy or toxic or a strange mix of both? Granik’s humane and lightly haunting film leaves us to answer that question, careful to show that society is a mixed bag, but confident that mere survival might only define one chapter of Tom’s life story.

Grade: B

“Leave No Trace” premiered at the 2018 Sundance Film Festival. It is currently seeking U.S. distribution.